624 



COMPARATIVE ANATOMY 

 Head Segments According to Beard 



In cyclostome embryos, Kupffer ('95) found thirteen such epibranchial 

 sense organs, five anterior to the facial ganghon and seven posterior. 

 Six of these are associated with the visceral branches of the vagus nerve. 

 Kupffer 's results, however, have never been confirmed, and relatively 



N. FACIALIS 

 N. TRIGEMINUS 

 ? 



N. GLOSSOPHARYNGtUS 

 EAR/ ,N. VAGUS /LATERAL LINE NERVE 



NOTOCHORD 



NOSE / 



EPIBRANCHIAL 



GANGLIA (x-an) 



HYPOPHYSIS 



Fig. 518.— A diagram of the head of a Petromyzon (ammocoetes) embryo seen from 

 the left side showing the series of epibranchial placodes in solid black. Beard and 

 Kupffer have suggested that such placodes or "epibranchial sense organs" might be 

 used as criteria of the primitive segmentation of the head. (Redrawn after Kupffer.) 



few schemes of cephalic segmentation make use of epibranchial ganglia 

 as criteria of metamerism. 



In the attempt to compare a head segment with a trunk segment, a 

 number of mistakes have been made. Gegenbaur's homology of a 

 skeletal visceral arch with a rib is invalidated by the fact that the two 



